Treat a group of friends as a single entity

[henryjchang]

Connected to #65 and #80.

The same way you're able to log a friend, you should be able to log a group of friends. The same person can show up in multiple groups and have different hangout frequencies. This enhancement does not require you to be able to identify that a single person belongs to multiple groups.

[marwahaha]

I find this similar to the "Tag friends" concept as in #50 . I implemented it but decided against merging it for the time being. If you'd like to review or adjust that code, please do!